Curable resin composition for fuel cell electrolyte film and electrolyte film, process for producing the same, electrolyte film/electrode assembly, and process for producing the same

ABSTRACT

A curable resin composition for fuel cell electrolyte films characterized by comprising (1) 100 parts by mass of a monomer having at least one ethylenically unsaturated group per molecule and having, per molecule, either at least one, tonically conductive group or at least one precursor group capable of giving an tonically conductive group through a chemical reaction, (2) 10-400 parts by mass of an oligomer which has, per molecule, at least two reactive groups copolymerizable with the ethylenically unsaturated group of the ingredient (1) and has a number-average molecular weight of 400 or higher, (3) 10-400 parts by mass of a fluororesin, and (4) 0-2,000 parts by mass of a solvent.

TECHNICAL FIELD

This invention relates to a curable resin composition for polymerelectrolyte fuel cell electrolyte films, an electrolyte film and itspreparation using the resin composition, and a fuel cell electrolytefilm/electrode assembly and its preparation.

BACKGROUND ART

Fuel cells using solid polymer electrolyte (SPE) films are expected tofind widespread practical use as power supplies for electric cars andsmall-size auxiliary power supplies due to a low operating temperaturebelow 100° C. and a high energy density. For such SPE fuel cells,constituent technologies relating to electrolyte films, platinum basecatalysts, gas diffusion electrodes, and electrolyte film/electrodeassemblies are important. Among others, the electrolyte films andelectrolyte film/electrode assemblies are one of the most importanttechnologies relating to the performance of fuel cells.

In SPE fuel cells, an electrolyte film on its opposite sides is combinedwith a fuel diffusion electrode and an air diffusion electrode so thatthe electrolyte film and the electrodes form a substantially integralstructure. Then the electrolyte film not only acts as an electrolyte forconducting protons, but also plays the role of a diaphragm forpreventing a fuel such as hydrogen or methanol from directly mixing withan oxidant such as air or oxygen even under applied pressure. From theelectrolyte aspect, the electrolyte film is required to have a high ion(proton) transfer velocity, a high ion exchange capacity, and a high andconstant water-retaining ability enough to maintain a low electricresistance. The role of a diaphragm, on the other hand, requires theelectrolyte film to have a high dynamic strength, dimensional stability,chemical stability during long-term service, and no extra permeation ofhydrogen gas or methanol as the fuel and oxygen gas as the oxidant.

Electrolyte films used in early SPE fuel cells were ion exchange filmsof hydrocarbon resins obtained through copolymerization of styrene withdivinyl benzene. These electrolyte films, however, lacked practicalusefulness due to very low durability. Thereafter, perfluorosulfonicacid-modified fluororesin films developed by E.I. duPont andcommercially available under the trade mark “Nafion” have been widelyused instead.

Conventional fluororesin base electrolyte films as typified by Nafionare improved in chemical durability and stability. However, when theyare applied to direct methanol fuel cells (DMFC) using methanol as thefuel, a crossover phenomenon that methanol runs through the electrolytefilm occurs, resulting in a reduced output. Another problem associatedwith conventional fluororesin base electrolyte films as typified byNafion is an increased cost because their manufacture starts from thesynthesis of monomers and requires a number of steps. This becomes asubstantial bar against practical applications. The ion conductivitymust be kept low in order to hold down the crossover of methanol. At thepresent, there is a trade-off between them. It remains unsolved toreduce the methanol crossover while maintaining a high ion conductivity.

With respect to the thickness of electrolyte films, as the film becomesthinner, proton conduction becomes easier and hence, fuel cells providebetter power generation characteristics. Thin electrolyte films,however, suffer from a problem that they can be ruptured when anelectrolyte film and electrodes are pressed together at elevatedtemperature to enhance the bond therebetween.

Efforts have been made to develop inexpensive electrolyte films that canreplace the Nafion and similar films. A number of electrolyte filmsunder study are described in Journal of Power Sources, 114 (2003), pp.32-53. However, these electrolyte films after their film formation arejoined to electrodes by pressing at elevated temperatures, which leavesproblems of possible rupture of films and complex steps. The joiningunder heat and pressure does not always achieve a sufficient adhesion.

To improve the level of productivity and adhesion, JP-A 2003-203646proposes to apply a solution of an electrolyte film in a solvent onto anelectrode, and press bond the assembly with the solvent partially lefttherein. Since the electrolyte film has not been cured, only lowadhesion is achieved.

JP-A 2003-217342 and JP-A 2003-217343 disclose crosslinking ofelectrolyte films for the purpose of improving durability. Since solidelectrolyte films are crosslinked, subsequent press bonding at elevatedtemperatures is necessary to fabricate an electrolyte film/electrodeassembly.

Also, WO 03/033576 discloses a method of controlling the fuelpermeability of an electrolyte film by impregnating the electrolyte filmwith a non-electrolyte monomer, followed by polymerization. Thenon-electrolyte monomer is cured. However, since the film subject toimpregnation is in solid form, subsequent press bonding at elevatedtemperatures is necessary.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

An object of the present invention, which has been made under theabove-described circumstances, is to provide curable resin compositionsfor forming fuel cell electrolyte films having excellent protonconduction and low methanol permeability when applied to DMFC;electrolyte films and electrolyte film/electrode assemblies; a methodfor producing electrolyte films at a high level of productivity; and amethod for manufacturing electrolyte film/electrode assemblies in whichan electrolyte film and electrodes are tightly joined without a need forhot pressing.

Means for Solving the Problem

Making extensive investigations to attain the above objects, theinventor has discovered that when a curable resin composition comprising(1) 100 parts by mass of a monomer containing per molecule at least oneethylenically unsaturated group and at least one ion conductive group orprecursor group capable of forming an ion conductive group throughchemical reaction, (2) 10 to 400 parts by mass of an oligomer containingper molecule at least two reactive groups copolymerizable with theethylenically unsaturated group in component (1) and having a numberaverage molecular weight of at least 400, (3) 10 to 400 parts by mass ofa fluororesin, and (4) 0 to 2,000 parts by mass of a solvent ispolymerized and cured by heating or ultraviolet or electron beamirradiation, the resulting cured film has excellent ion conductivity andsatisfactory elongation and strength; that this cured film is useful asthe electrolyte in a SPE fuel cell, i.e., an electrolyte film useful infuel cells as demonstrated by minimized permeability of methanol as thefuel in the case of DMFC can be manufactured at a high level ofproductivity; that by coating the curable resin composition onto a firstelectrode having a catalyst borne thereon, forming a cured film byheating or ultraviolet or electron beam irradiation, and disposing asecond electrode having a catalyst borne thereon contiguous to the curedfilm, or by coating the curable resin composition onto a first electrodehaving a catalyst borne thereon, disposing a second electrode having acatalyst borne thereon contiguous to the coated composition, and curingthe coated composition into a cured film by heating or electron beamirradiation, an electrolyte film/electrode assembly in which theelectrolyte film is tightly joined to the electrodes without a need forhot pressing and which is useful in fuel cells can be manufactured in anindustrially advantageous manner. The invention is predicated on thesefindings.

Accordingly, the present invention provides a curable resin compositionfor fuel cell electrolyte films, a method for producing a fuel cellelectrolyte film, and a method for manufacturing an electrolytefilm/electrode assembly, as defined below.

Claim 1:

A curable resin composition for fuel cell electrolyte films, comprising

(1) 100 parts by mass of a monomer containing per molecule at least oneethylenically unsaturated group and at least one ion conductive group orat least one precursor group capable of forming an ion conductive groupthrough chemical reaction,

(2) 10 to 400 parts by mass of an oligomer containing per molecule atleast two reactive groups copolymerizable with the ethylenicallyunsaturated group in component (1) and having a number average molecularweight of at least 400,

(3) 10 to 400 parts by mass of a fluororesin, and

(4) 0 to 2,000 parts by mass of a solvent.

Claim 2:

The fuel cell electrolyte film-forming curable resin composition ofclaim 1, wherein the ion conductive group in component (1) is a sulfonicacid group.

Claim 3:

The fuel cell electrolyte film-forming curable resin composition ofclaim 1 or 2, wherein the fluororesin as component (3) is selected fromthe group consisting of polytetrafluoroethylene,tetrafluoroethylene-hexafluoropropylene copolymers,tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers,polychlorotrifluoroethylene, ethylene-tetrafluoroethylene copolymers,polyvinylidene fluoride, polyvinyl fluoride, andtrifluoroethylene-ethylene copolymers.

Claim 4:

A fuel cell electrolyte film comprising

a cured film which is prepared by polymerizing (1) 100 parts by mass ofa monomer containing per molecule at least one ethylenically unsaturatedgroup and at least one ion conductive group or at least one precursorgroup capable of forming an ion conductive group through chemicalreaction and (2) 10 to 400 parts by mass of an oligomer containing permolecule at least two reactive groups copolymerizable with theethylenically unsaturated group in component (1) and having a numberaverage molecular weight of at least 400, followed by curing, with theproviso that when component (1) contains the precursor group, theprecursor group is converted into an ion conductive group, and

(3) 10 to 400 parts by mass of a fluororesin which is uniformlydispersed and incorporated in the cured film.

Claim 5:

A method for producing a fuel cell electrolyte film, comprising thesteps of:

coating the fuel cell electrolyte film-forming curable resin compositionof claim 1, 2 or 3, wherein the monomer as component (1) contains theion conductive group, onto a substrate to a buildup of up to 200 μm, and

curing the coated curable resin composition to form a cured film byheating or ultraviolet or electron beam irradiation.

Claim 6:

A method for producing a fuel cell electrolyte film, comprising thesteps of:

coating the fuel cell electrolyte film-forming curable resin compositionof claim 1, 2 or 3, wherein the monomer as component (1) contains theprecursor group capable of forming an ion conductive group, onto asubstrate to a buildup of up to 200 μm,

curing the coated curable resin composition to form a cured film byheating or ultraviolet or electron beam irradiation, and

converting the precursor groups into ion conductive groups.

Claim 7:

An electrolyte film/electrode assembly for fuel cells, comprising anelectrolyte film disposed between first and second electrodes eachhaving a catalyst borne thereon,

said electrolyte film comprising

a cured film prepared by polymerizing (1) 100 parts by mass of a monomercontaining per molecule at least one ethylenically unsaturated group andat least one ion conductive group or at least one precursor groupcapable of forming an ion conductive group through chemical reaction,and (2) 10 to 400 parts by mass of an oligomer containing per moleculeat least two reactive groups copolymerizable with the ethylenicallyunsaturated group in component (1) and having a number average molecularweight of at least 400, followed by curing, with the proviso that ifcomponent (1) contains the precursor group, the precursor group isconverted into an ion conductive group, and

(3) 10 to 400 parts by mass of a fluororesin uniformly dispersed andincorporated in the cured film.

Claim 8:

A method for manufacturing an electrolyte film/electrode assembly forfuel cells, comprising the steps of:

coating the fuel cell electrolyte film-forming curable resin compositionof claim 1, 2 or 3 onto a first electrode having a catalyst bornethereon,

curing the coated composition into a cured film by heating orultraviolet or electron beam irradiation, and

disposing a second electrode having a catalyst borne thereon on thecured film.

Claim 9:

A method for manufacturing an electrolyte film/electrode assembly forfuel cells, comprising the steps of:

coating the fuel cell electrolyte film-forming curable resin compositionof claim 1, 2 or 3 onto a first electrode having a catalyst bornethereon,

disposing a second electrode having a catalyst borne thereon on thecoated composition, and

curing the coated composition into a cured film by heating or electronbeam irradiation.

BENEFITS OF THE INVENTION

According to the invention, there are available an electrolyte film andan electrolyte film/electrode assembly for use in fuel cells whichsatisfy cell-related properties including high ion conductivity, lowmethanol permeability, and film strength as well as productivity at thesame time. The fuel cell electrolyte film produced by the method of theinvention can have a reduced thickness which leads to effective ionconduction and is thus quite useful as the electrolyte film in polymerelectrolyte fuel cells and especially direct methanol-air fuel cells.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view illustrating one typical method ofpreparing an electrolyte film/electrode assembly according to theinvention.

THE BEST MODE FOR CARRYING OUT THE INVENTION

The curable resin composition of the invention for forming a fuel cellelectrolyte film comprises (1) a monomer containing per molecule atleast one ethylenically unsaturated group and at least one ionconductive group or precursor group capable of forming an ion conductivegroup through chemical reaction, (2) an oligomer containing per moleculeat least two reactive groups copolymerizable with the ethylenicallyunsaturated group in component (1) and having a number average molecularweight of at least 400, (3) a fluororesin, and optionally, (4) asolvent.

Examples of the compound or monomer containing at least oneethylenically unsaturated group and at least one ion conductive group orprecursor group capable of forming an ion conductive group throughchemical reaction in a common molecule, used as component (1), includecarboxylic acid group-containing monomers such as (meth)acrylic acid;sulfonic acid group-containing monomers and alkali metal salts thereofsuch as acrylamide sulfonic acid, styrenesulfonic acid,allylbenzenesulfonic acid, allyloxybenzenesulfonic acid, vinylsulfonicacid, allylsulfonic acid, fluorovinylsulfonic acid, and perfluorovinylether sulfonic acid; phosphoric acid group-containing monomers such asmethacryloyloxyethyl phosphate; and examples of the compound free of ionconducting functional group (compound capable of imparting ionconduction by utilizing chemical reaction) include glycidyl(meth)acrylate monomers. Those monomers having a molecular weight ofless than 1,000 are desirable because the cured films therefrom havehigher ion conductivity. Suitable ion conductive groups includecarboxylic acid groups, sulfonic acid groups, phosphoric acid groups,and the like, with the sulfonic acid groups being preferred. Examples ofthe precursor group capable of forming an ion conductive group throughchemical reaction include acyloxy groups, ester groups (—COOR wherein Ris a monovalent hydrocarbon group), acid imide groups, halogenatedsulfonyl groups, glycidyl groups, and the like. The precursor groupundergoes chemical reaction with sodium hydroxide, methanol or sodiumsulfite, for example, to form a carboxylic acid or sulfonic acid group.

Examples of the oligomer containing per molecule at least two reactivegroups copolymerizable with the ethylenically unsaturated group incomponent (1) and having a number average molecular weight of at least400, used as component (2), include polyethylene glycoldi(meth)acrylate, di(meth)acrylate having a perfluoroalkyl etherstructure, polyethylene glycol, polypropylene glycol, polytetramethyleneglycol, polybutylene glycol, fluoroethylene glycol, polyetherpolyacrylates such as diurethane (meth)acrylate of a diol having aperfluoroalkyl ether structure, polyester polyacrylates, (meth)acryloxygroup-containing organopolysiloxanes, vinyl group-containingorganopolysiloxanes, and alkoxy group-containing organopolysiloxanes.Those oligomers having a number average molecular weight of at least 400are desirable because the resulting compositions are more curable.

Suitable reactive groups include ethylenically unsaturated groups andthey undergo radical polymerization with ethylenically unsaturatedgroups in component (1) to build up a molecular weight.

It is noted that the number average molecular weight is more preferably400 to 2,000, and even more preferably 800 to 1,000. The number averagemolecular weight is determined by gel permeation chromatography on thebasis of polystyrene.

No particular limit is imposed on the method of preparing the oligomerserving as component (2). In the case of polyurethane (meth)acrylateoligomers such as diurethane (meth)acrylate, they are preferablyprepared by reacting a polyol with a diisocyanate in a ratio OH/NCO<1,and further reacting with a compound having a functional group capableof reacting with residual isocyanate groups (e.g., hydroxyl group) andan acrylic group.

The polyurethane (meth)acrylate oligomers are described in more detail.They may be prepared through urethane-forming reaction of (a) a polyolcomponent, (b) a polyisocyanate component, and (c) a hydroxyl-containing(meth)acrylate compound. The polyurethane (meth)acrylate oligomers mayhave a number average molecular weight selected in the range of about400 to about 10,000, and preferably about 400 to about 5,000.

(a) Polyol Component

Examples of the polyol component include polyether polyols, polyesterpolyols, polycarbonate polyols, and alkyl diols, while fluorinated formsof the foregoing are also effectively used.

[Polyether Polyols]

Examples of suitable polyether polyols include homopolymers orcopolymers of alkylene oxides (typically C₂₋₅ alkylene oxides such asethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran,3-methyl-tetrahydrofuran), homopolymers or copolymers of the foregoingalkylene oxides using aliphatic C₁₂₋₄₀ polyols (e.g., 1,2-hydroxystearylalcohol, hydrogenated dimer diol) as an initiator, alkylene oxide (e.g.,propylene oxide, butylene oxide, tetrahydrofuran) adducts of bisphenolA, and alkylene oxide (e.g., propylene oxide, butylene oxide,tetrahydrofuran) adducts of hydrogenated bisphenol A. Fluorinated formsof the foregoing compounds are also preferably used. These polyetherpolyols may be used alone or in combination of two or more.

The preferred polyether polyols include homopolymers or copolymers ofC₂₋₄ alkylene oxides, especially C₃₋₄ alkylene oxides (specificallypropylene oxide and tetrahydrofuran), such as polyoxypropylene glycol,polytetramethylene ether glycol, and propylene oxide-tetrahydrofurancopolymers. Fluorinated forms of the foregoing compounds are alsopreferred. The polyether polyols may have a weight average molecularweight selected, for example, in the range of about 200 to about 10,000.

Commercially available products of the polyether polyols include, forexample,

-   (1) PEG600, PEG1000 and PEG2000 by Sanyo Chemical Industries, Ltd.    for polyethylene glycol,-   (2) Takelac P-21, Takelac P-22 and Takelac P-23 by Takeda Chemical    Industries, Ltd. for polyoxypropylene glycol,-   (3) PTG650, PTG850, PTG1000, PTG2000 and PTG4000 by Hodogaya    Chemical Co., Ltd. for polytetramethylene ether glycol,-   (4) ED-28 by Mitsui Toatsu Chemicals, Inc. and Excenol 510 by Asahi    Glass Co., Ltd. for propylene oxide-ethylene oxide copolymers,-   (5) PPTG1000, PPTG2000 and PPTG4000 by Hodogaya Chemical Co., Ltd.    for tetrahydrofuran-propylene oxide copolymers,-   (6) Unisafe DC-1100 and Unisafe DC-1800 by NOF Corp. for    tetrahydrofuran-ethylene oxide copolymers,-   (7) Uniol DA-400 and Uniol DA-700 by NOF Corp. for ethylene oxide    adducts of bisphenol A,-   (8) Uniol DB-400 by NOF Corp. for propylene oxide adducts of    bisphenol A, and-   (9) Perfluorotriethylene Glycol and Perfluorotetraethylene Glycol by    Exfluor Research Corp. and Fomblin Z DOL by Ausimont for    perfluoropolyether polyols.

[Polyester Polyols]

Examples of suitable polyester polyols include addition products of diolcompounds such as ethylene glycol, propylene glycol, diethylene glycol,dipropylene glycol, 1,5-pentaglycol, 3-methyl-1,5-pentane diol,1,6-hexane diol and neopentyl glycol to ε-caprolactam orβ-methyl-δ-valerolactone; reaction products of the foregoing diolcompounds with dibasic acids such as succinic acid, adipic acid,phthalic acid, hexahydrophthalic acid, and tetrahydrophthalic acid; andternary reaction products of the foregoing diol compounds, the foregoingdibasic acids, and ε-caprolactam or β-methyl-δ-valerolactone.

[Polycarbonate Polyols]

Examples of suitable polycarbonate polyols include diol compounds suchas 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol,1,4-butanediol, 1,5-octanediol, 1,4-bis(hydroxymethyl)cyclohexane,2-methylpropanediol, dipropylene glycol, dibutylene glycol, andbisphenol A; addition reaction products of the foregoing diol compoundswith 2 to 6 moles of ethylene oxide; and reaction products of theforegoing diol compounds with short-chain dialkyl carbonates such asdimethyl carbonate and diethyl carbonate.

Also useful are polyester diols in the form of addition reactionproducts of these polycarbonate polyols with ethylene oxide, propyleneoxide, ε-caprolactam or β-methyl-δ-valerolactone.

Commercially available products of the polycarbonate polyols includeDesmophene 2020E by Sumitomo Bayer Co., Ltd. and DN-980, DN-980, DN-982and DN-983 by Nippon Polyurethane Industry Co., Ltd.

[Alkyl Diols]

Examples of suitable alkyl diols include 1,6-hexanediol,3-methyl-1,5-pentanediol, neopentyl glycol, 1,4-butanediol,1,5-octanediol, 1,4-dihydroxycyclohexane,1,4-bis(hydroxymethyl)cyclohexane, 2-methylpropanediol, tricyclodecanedimethanol, 1,4-bis(hydroxymethyl)benzene, bisphenol A, andperfluoroalkyl diols of 6 to 12 carbon atoms.

Of these polyols, polyether polyols and alkyl diols are preferred for abalance of physical properties and durability of the resin of theinvention.

(b) Polyisocyanate Component

Examples of the polyisocyanate component include

-   diisocyanates such as tolylene diisocyanate,-   4,4′-diphenylmethane diisocyanate,-   hydrogenated 4,4′-diphenylmethane diisocyanate,-   xylylene diisocyanate, hydrogenated xylylene diisocyanate,-   hexamethylene diisocyanate, isophorone diisocyanate,-   1,5-naphthalene diisocyanate, tolidine diisocyanate,-   m-phenylene diisocyanate, p-phenylene diisocyanate,-   2,2,4-trimethylhexamethylene diisocyanate,-   2,4,4-trimethylhexamethylene diisocyanate,-   transcyclohexane-1,4-diisocyanate, lysine diisocyanate,-   tetramethylxylene diisocyanate, 1,4-cyclohexane diisocyanate,-   1,3-cyclohexane diisocyanate,-   1,4-bis[isocyanatomethyl]cyclohexane,-   methyl-2,4-cyclohexane diisocyanate,-   methyl-2,6-cyclohexane diisocyanate, and-   norbornene diisocyanate (or 1,3-cyclopentene diisocyanate);-   and polyisocyanates such as lysine ester triisocyanate,-   1,6,11-undecane triisocyanate,-   1,8-diisocyanato-4-isocyanatomethyloctane,-   1,3,6-hexamethylene triisocyanate,-   bicycloheptane triisocyanate,-   trimethylhexamethylene diisocyanate,-   1,3,5-triisocyanatocyclohexane,-   1,3,5-trimethylisocyanatocyclohexane,-   2-[3-isocyanatopropyl]-2,5-di[isocyanatomethyl]-bicyclo-[2,2,1]heptane,-   2-[3-isocyanatopropyl]-2,6-di[isocyanatomethyl]-bicyclo-[2,2,1]heptane,-   3-[3-isocyanatopropyl]-2,5-di[isocyanatomethyl]-bicyclo-[2,2,1]heptane,-   5-[2-isocyanatoethyl]-2-isocyanatomethyl-3-[3-isocyanato-propyl]-bicyclo[2,2,1]heptane,-   6-[2-isocyanatoethyl]-2-isocyanatomethyl-3-[3-isocyanato-propyl]-bicyclo[2,2,1]heptane,-   5-[2-isocyanatoethyl]-2-isocyanatomethyl-2-[3-isocyanato-propyl]-bicyclo[2,2,1]heptane,    and-   6-[2-isocyanatoethyl]-2-isocyanatomethyl-2-[3-isocyanato-propyl]-bicyclo[2,1,1]heptane.    These diisocyanates may be used alone or in admixture.

Among others, 2,4-tolylene diisocyanate and isophorone diisocyanate arepreferred for ease of synthesis reaction and a balance of cured filmproperties.

(c) Hydroxyl-Containing (meth)acrylate Compound

Examples of suitable hydroxyl-containing (meth)acrylate compoundsinclude hydroxyalkyl (meth)acrylates (e.g., hydroxy-C₂₋₁₀ alkyl(meth)acrylates such as

-   2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,-   3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate,-   4-hydroxybutyl (meth)acrylate, pentanediol mono(meth)acrylate,-   hexanediol mono(meth)acrylate, and-   neopentyl glycol mono(meth)acrylate),-   2-hydroxy-3-phenyloxypropyl (meth)acrylate,-   2-hydroxyalkyl (meth)acryloyl phosphates,-   4-hydroxycyclohexyl (meth)acrylate, cyclohexane-1,4-dimethanol    mono(meth)acrylate,-   trimethylol propane di(meth)acrylate,-   pentaerythritol tri(meth)acrylate, etc., as well as compounds    produced through addition reaction of glycidyl or epoxy    group-containing compounds (e.g., alkyl glycidyl ethers, allyl    glycidyl ethers, glycidyl (meth)acrylates) to (meth)acrylic acid.    These hydroxyl-containing (meth)acrylate compounds may be used alone    or in combination of two or more. Preferred hydroxyl-containing    (meth)acrylate compounds are hydroxy-C₂₋₄ alkyl (meth)acrylates,-   specifically 2-hydroxyethyl (meth)acrylate and-   2-hydroxypropyl (meth)acrylate.

The polyurethane (meth)acrylate oligomers can be prepared by reactingthe foregoing components. The components which constitute polyurethane(meth)acrylate oligomers are combined, for example, in such a proportionthat approximately 0.1 to 0.8 mole, preferably 0.2 to 0.7 mole, morepreferably 0.2 to 0.5 mole of hydroxyl groups on the polyol componentand approximately 0.2 to 0.9 mole, preferably 0.3 to 0.8 mole, morepreferably 0.5 to 0.8 mole of the hydroxyl-containing (meth)acrylate areavailable per mole of isocyanate groups on the polyisocyanate.

It is not particularly limited how to react the above-describedcomponents. The components may be mixed altogether for reaction.Alternatively, the polyisocyanate may be reacted with either one of thepolyol component and the hydroxyl-containing (meth)acrylate and thenwith the other.

Catalysts which can be used in the urethane-forming reactions includeorganometallic urethane-forming catalysts such as stannous octoate,dibutyltin diacetate, dibutyltin dilaurate, cobalt naphthenate, and leadnaphthenate; and amine catalysts such as triethylamine, triethylenediamine, and diazabicycloundecene, while other well-knownurethane-forming catalysts may also be used.

Examples of the fluororesin which is used herein as component (3) forthe purpose of suppressing the methanol permeability of the cured filminclude

-   polytetrafluoroethylene (PTFE),-   tetrafluoroethylene-hexafluoropropylene copolymers (FEP),-   tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA),-   polychlorotrifluoroethylene (PCTFE),-   ethylene-tetrafluoroethylene copolymers (ETFE),-   polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and-   trifluoroethylene-ethylene copolymers (ECTFE), which may be used    alone or in admixture. It is noted that the fluororesins used herein    may be commercially available products having a number average    molecular weight of about 100,000 to about 600,000.

In the electrolyte film-forming curable resin composition of theinvention, a solvent may be used as component (4). The preferredsolvents used herein are those in which the ion conductive monomer ascomponent (1) and the oligomer as component (2) are uniformlydissolvable. Examples include ketones such as acetone and methyl ethylketone, esters such as ethyl acetate and butyl acetate, ethers such astetrahydrofuran and dioxane, N,N-dimethylformamide,N,N-dimethylacetamide, dimethyl sulfoxide, aromatic hydrocarbons such asbenzene and toluene, aliphatic and alicyclic hydrocarbons such asn-heptane, n-hexane and cyclohexane, and mixtures thereof. Inter alia,polar solvents are more preferable.

In the electrolyte film-forming curable resin composition of theinvention, the above-described components are compounded in such amountsthat the oligomer (2) is 10 to 400 parts by mass, desirably 20 to 100parts by mass, and more desirably 25 to 75 parts by mass per 100 partsby mass of the monomer (1). Less than 10 parts by mass of the oligomermay compromise curability whereas more than 400 parts by mass maysometimes lead to a decline of ion conductivity. The fluororesin (3) iscompounded in an amount of 10 to 400 parts by mass, desirably 40 to 130parts by mass, and more desirably 75 to 125 parts by mass per 100 partsby mass of the monomer (1) whereby the cured film can be reduced inpermeability of methanol as the fuel without detracting from its ionconductivity. With less than 10 parts by mass of the fluororesincompounded, the effect of suppressing methanol permeation may possiblybe reduced whereas more than 400 parts by mass may possibly detract fromion conductivity.

From the coating aspect, the composition should desirably have aviscosity at 25° C. of equal to or less than 100,000 mpa-s, and moredesirably 100 to 10,000 mPa-s. A composition with a viscosity of greaterthan 100,000 mPa-s may have poor leveling property and become difficultto form an even, thin coat whereas a composition with a viscosity ofless than 100 mpa-s may cause more cissing and become more penetrableinto a substrate. The amount of the solvent (4) compounded is determinedfrom the above and other aspects, and is usually 0 to 2,000 parts bymass, preferably 50 to 1,500 parts by mass, and more preferably 100 to1,000 parts by mass per 100 parts by mass of the monomer (1).

In the curable resin composition of the invention, an additional monomernot containing an ion conductive group or a precursor group thereof maybe included for the purposes of tailoring the elongation, strength,Young's modulus, and glass transition temperature of a cured film, orthe like. Suitable additional monomers include styrene, t-butylstyrene,n-lauryl acrylate, 2-ethylhexyl acrylate, n-hexyl acrylate, isooctylacrylate, 2-phenoxyethyl acrylate, and 2-ethoxyethyl acrylate. Thecombined use of additional monomer is acceptable as long as it does notsignificantly compromise the ion conduction of a cured film.

In the inventive composition, heteropolyacids such as phospho-tungstatemay be added for the purpose of improving ion conduction. Also,inorganic compounds such as oxides, nitrides or carbides may be added asthe filler for the purposes of preventing hydrogen, alcohol, water oroxygen from permeating through the fuel cell. Exemplary fillers includeboron nitride, silicon carbide and silica.

The curable resin composition of the invention can be applied onto afilm substrate such as polyester film, polypropylene film, polyethylenefilm or tetrafluoroethylene film and heated or irradiated withultraviolet radiation (UV) or electron beams (EB) for thereby forming acured film. The cured film desirably has a thickness of up to 200 μm,and more desirably 1 to 50 μm. A film of more than 200 μm has a greaterfilm resistance when used as the electrolyte film in a fuel cell,leading to a reduced output. A film of less than 1 μm may provide a lessbarrier to hydrogen gas or methanol as the fuel in the fuel cell,leading to a reduced output.

For curing the curable resin composition of the invention, thecomposition is preferably heated at a temperature of at least 80° C.,more preferably at least 100° C. The upper limit of the heatingtemperature is selected as appropriate while it is preferably up to 150°C., especially up to 120° C., from the standpoint of heat resistance ofcurable resin. Although the heating time varies with the heatingtemperature, it is usually 1 minute to 2 hours, especially 3 minutes to30 minutes. In an alternative embodiment, UV is irradiated in anexposure dose of at least 10 mJ/cm², or EB is irradiated so as toprovide an absorbed dose of at least 5 kGy. In the case of UV curing, anappropriate exposure dose is 10 to 1,000 mJ/cm², and more desirably 50to 500 mJ/cm². An exposure dose of less than 10 mJ/cm² may cause thecurable resin to cure short whereas an exposure dose in excess of 1,000mJ/cm² is uneconomical because of an energy waste and a loss ofproduction efficiency. In the case of EB curing, the absorbed dose isdesirably 5 to 500 kGy, and more desirably 10 to 100 kGy because anabsorbed dose of less than 5 kGy may lead to undercure and an absorbeddose in excess of 500 kGy may cause decomposition of the curable resin.

For helping the composition cure, heat polymerization initiators such asazobisisobutyronitrile in the case of heat curing, andphoto-polymerization initiators such as benzophenone in the case of UVcuring may be additionally used. Once a coating of the composition iscured by heating, it may be further cured by irradiation of UV or EB.

The temperature at which UV or EB is irradiated may be around roomtemperature. In order to adjust the viscosity of the resin compositionso that the composition may be effectively coated, and to produce acoating thereof with a consistent thickness and a consistent surfacestate, the resin composition or the irradiating atmosphere may becontrolled in advance to a certain temperature. Desirably, the resincomposition and the irradiating atmosphere are controlled to a constanttemperature in the range of 25 to 60° C.

The atmosphere in which the curable resin composition is cured ispreferably an inert gas atmosphere such as nitrogen, helium or argon sothat radical polymerization may readily take place. The gas shouldpreferably have an oxygen concentration of up to 500 ppm, morepreferably up to 200 ppm.

As described above, an electrolyte film can be manufactured by coatingthe fuel cell electrolyte film-forming curable resin composition of theinvention onto a substrate to a buildup equal to or less than 200 μm,and curing the coated composition into a cured film by heating or UV orEB irradiation. In the embodiment wherein the monomer (1) has an ionconductive precursor group rather than an ion conductive group, theprecursor groups in the cured film must be converted to ion conductivegroups by suitable treatment, for example, alkali-assisted hydrolysis orreaction with sodium sulfite.

The curing process as described above yields an electrolyte film inwhich components (1) and (2) are copolymerized and cured and thefluororesin as component (3) is uniformly dispersed and incorporated inthe cured film.

The electrolyte film for fuel cells according to the invention isdisposed contiguous to and between first and second electrodes eachhaving a catalyst borne thereon to form an electrolyte film/electrodeassembly for fuel cells. Specifically, the electrolyte film/electrodeassembly is prepared by either of the following:

method (i) involving applying an electrolyte film-forming curable resincomposition having ion conductivity onto a first electrode having acatalyst borne thereon to form a coating, curing the coating into acured film by heating or UV or EB irradiation, and disposing a secondelectrode having a catalyst borne thereon contiguous to the cured film,and

method (ii) involving applying an electrolyte film-forming curable resincomposition having ion conductivity onto a first electrode having acatalyst borne thereon to form a coating, disposing a second electrodehaving a catalyst borne thereon contiguous to the uncured coating, andcuring the curable resin into a cured film by heating or EB irradiation.

Referring to FIG. 1, method (ii) is illustrated. An air electrode 1includes a catalyst layer 3 coated on a carbon paper 2. Similarly, afuel electrode 4 includes a catalyst layer 6 coated on a carbon paper 5.7 denotes a coating of the electrolyte film-forming curable resincomposition (or an electrolyte film resulting from curing thereof). Forexample, the assembly is manufactured by forming the coating 7 on thecatalyst layer 6 of the fuel electrode 4, placing the air electrode 1thereon such that the catalyst layer 3 is contiguous to the coating 7,and then heating or irradiating EB for curing the coating 7, yielding acured film or electrolyte film.

The electrode having a catalyst borne thereon may be a conventional fuelcell electrode (either fuel electrode or air electrode) on which acatalyst is borne. The construction and material of the electrode may beselected from those well known for fuel cells. The catalyst may also beselected from those well known for fuel cells, for example, platinumbase catalysts.

In the above method, a coating of the composition or an electrolyte filmis joined to electrodes by compression bonding under about 0.05 to 5kG/cm² using a press or the like. A firm bond is established between theelectrolyte film and the electrodes without a need for hot pressing.

The electrolyte film and the electrolyte film/electrode assemblyaccording to the invention are advantageously used in fuel cells. Thefuel cell includes a fuel electrode, an air electrode, and a SPE film inthin film form disposed therebetween and tightly bonded thereto. Acatalyst layer, a fuel diffusion layer and a separator are disposed onboth sides of the SPE film to construct a fuel cell having improvedpower generation.

EXAMPLE

Examples of the invention are given below together with ComparativeExamples by way of illustration and not by way of limitation. It isunderstood that the number average molecular weight (Mn) is measured byhigh-performance GPC system HLC-8220 (Tosoh Corp.), and the viscosity ismeasured by a rotational viscometer.

Example 1

A reactor was charged with 100 g of fluorotetraethylene glycol having aMn of 410 and 0.05 g of 2,6-di-tert-butylhydroxytoluene (BHT). In anitrogen stream at 65-70° C., 84.9 g of 2,4-tolylene diisocyanate wasadded dropwise to the reactor. After the completion of dropwiseaddition, the reaction continued at 70° C. for a further 2 hours,followed by addition of 0.02 g of dibutyltin dilaurate. In dry air, 56.6g of 2-hydroxyethyl acrylate was added dropwise. The reaction continuedat 70° C. for a further 5 hours, yielding a fluoropolyether urethaneacrylate oligomer having a Mn of 990 (Oligomer A).

50 g of Oligomer A was mixed with 100 g ofacrylamidemethylpropanesulfonic acid, 100 g of polyvinylidene fluoride(PVDF) powder having a Mn of 543,000, and 900 g of N,N-dimethylformamide(DMF) as a solvent to form a curable resin composition B of clearfluorescent color having a viscosity of 8,000 mpa-s at 25° C.

Next, using an applicator, the curable resin composition B was appliedonto a glass plate to a buildup of 50 μm. In a nitrogen atmospherehaving an oxygen concentration of up to 50 ppm, EB irradiation wasperformed at an accelerating voltage of 300 kV so as to provide anabsorbed dose of 50 kGy, yielding a cured film.

The film was immersed in deionized water at 25° C. for 24 hours, afterwhich it was taken out and wiped on the surface with gauze. Using animpedance/gain-phase analyzer 1260 (Schulumberger Technologies) andplatinum plates as the electrodes, a proton conductivity at 25° C. wasmeasured to be 0.10 S/cm. By a gas chromatography analyzer, the film wasmeasured for permeability of a 1M methanol aqueous solution at 25° C.,finding a permeability of 0.07 kg/m²-h.

Example 2

A 5% isopropyl alcohol solution of Nafion (Aldrich) and carbon having20% by mass of platinum borne thereon, Vulcan XC-72 (Cabot) were kneadedto form a paste. Using a wire bar, this catalyst paste was applied ontoa carbon paper TGP-H-090 (Toray Co., Ltd.) so as to give a coatingweight of 0.34 mg/cm² of Pt catalyst. The coating was dried in a hot aircirculating dryer at 120° C. for 5 minutes, forming an electrode (fuelelectrode).

Using an applicator, the curable resin composition B was applied ontothis electrode to a buildup of about 30 μm. An electrode (air electrode)which was similarly constructed as the above electrode (fuel electrode)was laid on the coating. The laminate was press bonded by moving aroller at 5 kG/cm² and room temperature over two back and forth strokes.Using an electron beam-emitting system, the laminate was irradiated, inthe way illustrated in FIG. 1, with electron beams in a nitrogenatmosphere having an oxygen concentration of up to 50 ppm, at anaccelerating voltage of 300 kV and an absorbed dose of 50 kGy. Thecurable resin composition effectively cured, and the cured filmexhibited a firm bond to both the electrodes.

The film had an ion conductivity of 0.10 S/cm at 25° C. A cell using 1Mmethanol fuel at 30° C. produced an output of 20 mW/cm² at a currentflow of 100 mA/cm².

Comparative Example 1

150 g of Oligomer A, prepared in Example 1, was mixed with 100 g ofacrylamidemethylpropanesulfonic acid and 400 g of N,N-dimethylformamide(DMF) as a solvent to form a curable resin composition C having aviscosity of 10 mPa-s at 25° C. As in Example 1, a cured film wasproduced and its proton conductivity at 25° C. was measured to be 0.10S/cm. By a gas chromatography analyzer, the film was measured forpermeability of a IM methanol aqueous solution at 25° C., finding apermeability of 0.40 kg/m²-h.

Comparative Example 2

A film was produced as in Example 2 aside from using the curable resincomposition C of Comparative Example 1. The film had an ion conductivityof 0.10 S/cm at 25° C. A cell using 1M methanol fuel at 30° C. producedan output of 9 mW/cm² at a current flow of 100 mA/cm².

1-9. (canceled)
 10. A curable resin composition for fuel cellelectrolyte films, comprising (1) 100 parts by mass of a monomercontaining per molecule at least one ethylenically unsaturated group andat least one ion conductive group or at least one precursor groupcapable of forming an ion conductive group through chemical reaction,(2) 10 to 400 parts by mass of an oligomer containing per molecule atleast two reactive groups copolymerizable with the ethylenicallyunsaturated group in component (1) and having a number average molecularweight of at least 400, (3) 10 to 400 parts by mass of a fluororesin,and (4) 0 to 2,000 parts by mass of a solvent.
 11. The fuel cellelectrolyte film-forming curable resin composition of claim 1, whereinthe ion conductive group in component (1) is a sulfonic acid group. 12.The fuel cell electrolyte film-forming curable resin composition ofclaim 1, wherein the fluororesin as component (3) is selected from thegroup consisting of polytetrafluoroethylene,tetrafluoroethylene-hexafluoropropylene copolymers,tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers,polychlorotrifluoroethylene, ethylene-tetrafluoroethylene copolymers,polyvinylidene fluoride, polyvinyl fluoride, andtrifluoroethylene-ethylene copolymers.
 13. A fuel cell electrolyte filmcomprising a cured film which is prepared by polymerizing (1) 100 partsby mass of a monomer containing per molecule at least one ethylenicallyunsaturated group and at least one ion conductive group or at least oneprecursor group capable of forming an ion conductive group throughchemical reaction, and (2) 10 to 400 parts by mass of an oligomercontaining per molecule at least two reactive groups copolymerizablewith the ethylenically unsaturated group in component (1) and having anumber average molecular weight of at least 400, followed by curing,with the proviso that when component (1) contains the precursor group,the precursor group is converted into an ion conductive group, and (3)10 to 400 parts by mass of a fluororesin which is uniformly dispersedand incorporated in the cured film.
 14. A method for producing a fuelcell electrolyte film, comprising the steps of: coating the fuel cellelectrolyte film-forming curable resin composition of claim 1, whereinthe monomer as component (1) contains the ion conductive group, onto asubstrate to a buildup of up to 200 μm, and curing the coated curableresin composition to form a cured film by heating or ultraviolet orelectron beam irradiation.
 15. A method for producing a fuel cellelectrolyte film, comprising the steps of: coating the fuel cellelectrolyte film-forming curable resin composition of claim 1, whereinthe monomer as component (1) contains the precursor group capable offorming an ion conductive group, onto a substrate to a buildup of up to200 μm, curing the coated curable resin composition to form a cured filmby heating or ultraviolet or electron beam irradiation, and convertingthe precursor groups into ion conductive groups.
 16. An electrolytefilm/electrode assembly for fuel cells, comprising an electrolyte filmdisposed between first and second electrodes each having a catalystborne thereon, said electrolyte film comprising a cured film prepared bypolymerizing (1) 100 parts by mass of a monomer containing per moleculeat least one ethylenically unsaturated group and at least one ionconductive group or at least one precursor group capable of forming anion conductive group through chemical reaction, and (2) 10 to 400 partsby mass of an oligomer containing per molecule at least two reactivegroups copolymerizable with the ethylenically unsaturated group incomponent (1) and having a number average molecular weight of at least400, followed by curing, with the proviso that if component (1) containsthe precursor group, the precursor group is converted into an ionconductive group, and (3) 10 to 400 parts by mass of a fluororesinuniformly dispersed and incorporated in the cured film.
 17. A method formanufacturing an electrolyte film/electrode assembly for fuel cells,comprising the steps of: coating the fuel cell electrolyte film-formingcurable resin composition of claim 1 onto a first electrode having acatalyst borne thereon, curing the coated composition into a cured filmby heating or ultraviolet or electron beam irradiation, and disposing asecond electrode having a catalyst borne thereon on the cured film. 18.A method for manufacturing an electrolyte film/electrode assembly forfuel cells, comprising the steps of: coating the fuel cell electrolytefilm-forming curable resin composition of claim 1 onto a first electrodehaving a catalyst borne thereon, disposing a second electrode having acatalyst borne thereon on the coated composition, and curing the coatedcomposition into a cured film by heating or electron beam irradiation.